Because of its excellent material properties, heretofore, polysilicon has been the chief material used in the formation of integrated circuits.
In order to take advantage of the emerging technology of faster and smaller integrated circuit devices, it has become necessary to develop new materials which can be used as interconnection and gate materials instead of, or in conjunction with, polysilicon. These materials must have lower resistivity than polysilicon and should be compatible with current integrated circuit processes. Silicides of refractory metals (Mo, W. Ti and Ta) with their metallic conductivity and high temperature stability, meet these requirements.
One such silicide, in particular, titanium silicide (TiSi.sub.2) has been used as gate, contact and interconnect metallization in integrated circuit technologies due to its low resistivity and the low annealing temperature required to form the stable, low resistivity silicide phase. TiSi.sub.2 has found wide applications in the self-aligned silicide (salicide) technology [C. M. Osburn, M. Y. Tsai, S. Roberts, C. J. Lucchese and C. Y. Ting, VLSI Science and Technology/1982, edited by C. J. Dell'Oca and W. M. Bullis (Electrochem. Soc. Pennington, N.J., 1982), 82-7, p. 213 and C. Y. Ting. S. S. Iyer, C. M. Osburn C. J. Hu and A. M. Schweighart, VLSI Science and Technology/1982, edited by C. J. Dell'Oca and W. M. Bullis (Electrochem. Soc Pennington, N.J. 1982) 82-7, p. 224]. In the salicide process Ti is deposited by a Physical Vapor Deposition technique onto a patterned wafer. A two-step annealing process, which prevents bridging between the source/drain and the gate in a Metal-Oxide-Semiconductor transistor is then used to form titanium silicide in the regions where silicon is exposed. The unreacted metal on the oxide is then selectively removed by wet chemical etching.
Rosler et al (U.S. Pat. No. 4,557,943 issued 12/10/85) discloses a plasma enhanced CVD (PECVD) process for depositing TiSi.sub.2 films from SiH.sub.4 and
TiCl.sub.4 and plasma. At column 3, lines 32-41, they note that:
"It has been discovered that the sheet resistance of a given titanium silicide deposition is lower over silicon than over oxide, perhaps due to silicon incorporation during the anneal process. Also, the titanium silicide is more likely to be hazy or peel when deposited over oxide. To optimize the deposition for the microelectronic application, it has been found expedient to deposit a thin layer--e.g. 300 to 600 .ANG.--of amorphous silicon prior to the silicide deposition."
Rosler and Engle in J. Vac. Sci. Technol. 8, 2(4) 733 (1984), also report that the as-deposited silicide films were amorphous and an annealing temperature of 600-650.degree. C. brought the resistivity down to about 15-20 micro-ohms-cm. A small amount of chlorine was detected in the as-deposited films, which after sintering, was not detectable.
Pintchovski in U.S Pat. No. 4,619,038 issued 28 Oct. 1986 teaches selective formation of TiSi.sub.2 by high temperature 700.degree. -1000.degree. C. LPCVD of a titanium halide gas and an excess of hydrogen. Pintchovski's reaction takes place "in the absence of a silicon bearing gas". Pintchovski's reaction is: EQU "2TiCl.sub.4 (g)+Si(s).fwdarw.2TiCl.sub.2 (s)+SiCl.sub.4 (g)(1) EQU or EQU TiCl.sub.2 (s)+2Si.sub.2 (s)+H.sub.2 (g).fwdarw.TiSi.sub.2 (s)+2HCl(g)(2)"
Hieber et al. (U.S. Pat. No. 4,501,769, issued 2/26/85 claiming priority in Germany to 3/30/82) teaches selective deposition of high melting point (HMP) metal silicides (TaSi.sub.2) on Si surfaces by CVD of gaseous Si (SiH.sub.2 Cl.sub.2) and a HMP halogen (e g., TaCl.sub.5). Hieber et al. postulates that:
"The following considerations may, at least in part, explain the substrate-associated, and, thus, the selective deposition of the metal silicides:
On the basis of thermo-dynamic calculations, tantalum disilicide, in order to be formed in accordance with the equation: EQU TaCl.sub.5 +2SiH.sub.2 Cl.sub.2 +2.5H.sub.2 .fwdarw.TaSi.sub.2 +9HCl
must be capable of formation at temperatures around 800.degree. C., independently of the substrate material. The fact that, with the inventive method, the foregoing reaction does not occur on, for example, SiO.sub.2, or occurs only very slowly thereon, could be connected to the fact that the reaction gas mixture releases an excess of, for example, chlorine, or hydrogen chloride, which inhibits the course of the reaction in the described manner. Hydrogen disassociating at the silicon surface could contribute to the formation of monosilane so that a reaction in accordance with the following equation: EQU TaCl.sub.5 +2SiH.sub.4 .fwdarw.TaSi.sub.2 +5HCl+1.5H.sub.2
occurs at the silicon surface. The foregoing reaction is extremely favored thermo-dynamically and could thus explain the high selectivity of HMP metal silicide deposition on silicon surface areas in accordance with the principles of the invention."
Hieber et al. includes molybdenum, tungsten and tantalum in the group of HMP metals, but does not mention titanium.
Kemper et al., Extended Abstracts, Electrochem. Soc. Fall Meeting (New Orleans 1984), 84-2, p. 533, reports on low pressure CVD and Plasma enhanced CVD of TiSi.sub.2 on bare Si wafers, wafers covered with thermal SiO.sub.2 or LPCVD Si.sub.3 N.sub.4.
Kemper et al investigated the LPCVD of titanium silicide in a temperature range of 600.degree.-800.degree. C., a pressure range of 0.1-5 Torr, and a TiCl.sub.4 /SiH.sub.4 or TiCl.sub.4 /SiH.sub.2 Cl.sub.2 ratio of 0.1-10. They observed that LPCVD titanium silicide did not nucleate using only TiCl.sub.4 and SiH.sub.4 ; instead, an in-situ deposited polysilicon layer prior to the silicide deposition was necessary for the nucleation of titanium silicide. They concluded that any native oxide inhibited the deposition of titanium silicide. They also indicated that the surface roughness of the LPCVD silicide films deposited in their reactor was unacceptable, and speculated that a better vacuum system would have yielded smooth films.
Kemper et al. also experimented with the PECVD of titanium silicide. The PECVD films were deposited in a parallel plate experimental reactor with the wafers placed on the ground electrode. The following deposition conditions were used: deposition temperatures of 300.degree. and 350.degree. C., TiCl.sub.4 /SiH.sub.4 ratios of 1-2, and a frequency of 300 kHz. The as-deposited films were amorphous and required an annealing temperature of 750.degree. C. for 1 hour to bring down the resistivity to 20 micro-ohms-cm; the as-deposited PECVD titanium silicide films were reported to be smooth.
Gupta et al used an excimer laser (ArF, 193nm) with a focused cross-sectional area of 15.times.l mm to investigate the Laser-Induced CVD of titanium silicide. This is a gas phase reaction process that is initiated photochemically. The laser beam was passed 2 mm above the wafer surface. They were able to deposit titanium silicide films at substrate temperatures above 350.degree. C. The as-deposited films had high resistivities and were mostly amorphous, and required an annealing temperature of 650.degree.-700.degree. C. to reduce the resistivity to 20-30 micro-ohms-cm. The films were also contaminated with oxygen (detected by Auger) and chlorine (detected by RBS). Increasing the substrate deposition temperature resulted in rough silicide films; however, the silicide adhesion improved.
Tedrow et al., Appl. Phys. Lett. 1, 1985, report in-situ deposition of sequential films of polysilicon and titanium silicide wherein the as-deposited films had resistivities of 22-39 micro-ohms-cm from LPCVD of SiH.sub.4 and TiCl.sub.4. They report that:
"Deposition of polysilicon prior to the deposition of the silicide film has two advantages. Firstly, a polycide (polysilicon-silicide layered structure) film is obtained which preserves the nearly ideal Metal-Oxide-Semiconductor (MOS) characteristics of the underlying polysilicon; and secondly, the underlying polysilicon film provides a clean interface for the titanium silicide deposition."
Bouteville et al., J. Electrochem. Soc. 134(8), 2080 (1987), is the only known reported LPCVD work on the selective deposition of TiSi.sub.2. They use hydrogen reduction of titanium tetrachloride (TiCl.sub.4) in a temperature range of 700.degree.-1000.degree. C. and a total pressure of 100 Pa (0.75 Torr) to selectively deposit titanium on the silicon regions and form titanium silicide through silicon diffusion in the growing titanium film. However, their selective deposition process is not compatible with the fabrication of shallow junctions since the silicide is formed by supplying the silicon from the substrate. The resistivity of their titanium silicide films is high, 40-100 micro-ohms-cm, and they also refer to nucleation problems in the formation of titanium silicide films.
Consequently, despite extended efforts over a number of years by numerous scientists, a need still exists for an LPCVD selective TiSi.sub.2 deposition process which does not rely completely on supplying the silicon from the substrate and which reliably selectively produces high conductivity or low resistivity films.